First, since you are aware of this situation, take preventative steps; don't go out to your car alone; see if extra lighting can be provided, or possibly even cameras or other deterrents.
Second, I agree with those who say do as much as needed to get the person off you and unable to follow (or at least unable to follow more quickly than you can leave) and then run, screaming loudly. People who attack you do not want attention called to the fact; they want to mug and/or rape you quickly and get away - screaming will prevent that.
Third, remember that every situation is different. No advice you are given will be able to cover every eventuality, and as long as you, and whomever you are protecting, emerge reasonably unhurt, the method you use doesn't matter - it's the results that are important.
Second, I agree with those who say do as much as needed to get the person off you and unable to follow (or at least unable to follow more quickly than you can leave) and then run, screaming loudly. People who attack you do not want attention called to the fact; they want to mug and/or rape you quickly and get away - screaming will prevent that.
Third, remember that every situation is different. No advice you are given will be able to cover every eventuality, and as long as you, and whomever you are protecting, emerge reasonably unhurt, the method you use doesn't matter - it's the results that are important.